A PWM driver attempts to maintain a desired current level through a load by modulating the duty cycle of a direct current (DC) source being switched at a designated frequency. A PWM driver controller selectively supplies low amperage trigger signals to a switched power circuit, for example to the gate of a N-channel MOSFET transistor electrically connected in series with the load and DC source, such as a battery (BATT). A sensed value representative of the current flowing through the load is in turn fed back to the controller, in order to enable the switch to be turned off when the current is above the desired level and turned on when below the desired level.
One characteristic of a PWM driver is that while current to the load may be switched off at any time, the controller only permits current to be switched on to the load at predetermined intervals. Consequently, the frequency at which electrical current is supplied to the load is relatively constant, while the average current supplied to the load is dependent on the duty cycle or "pulse width" of the time during which the controller keeps the switch on.
Most inductive loads behave best if they are switched at the highest available frequency, because there is less "ripple" as the current rises and falls during successive on/off periods. It is important however, to minimize electromagnetic interference (EMI) introduced by current spikes on the battery line. Such spikes are created by a flyback diode included in the external driver circuit to recover energy when the inductor is disconnected from battery. It is particularly important to minimize flyback spikes when multiple PWM drivers share common battery lines, to avoid corrupting the feedback signals used by the other PWM driver controls in determining whether to turn on.
The present invention is directed to overcoming one or more of the disadvantages set forth above.